Wave transmission circuits



1380- 1939- w. VAN ROBERTS WAVE TRANSMISSION CIRCUITS I Filed Sept. 50, 1936 2 Sheets-Sheet l HIGH PASS FILTER WIMPED/WCE BAND P456 FILTER ERTS INVENTOR HlG/l/MPEDANLE WALTER VA 3.9.05

BAND ms: BY Lg F/L TER A'ITORNEY D60 1939- w. VAN B. ROBERTS 2,184,400

WAVE TRANSMISSION CIRCUITS Filed Sept. 30, 1936 2 Sheets-Sheet 2 s m E m MN I mm w N R w m m K A H M Y B Q m+ MN EQRQ E E m Q v GEE .w m uu$ M safi IL A" QM.) 2.50

ERR k Nmkm E ,ductances,

Patented Dec. 26, 1939 UNITED STATES PATENT q-OFFlCiE WAVE TRANSIVHSSION CIRCUITS Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a'corporation.

of Delaware Application September 30, 1936, SerialNo. 103,230 b 8 Claims.

justing the biasing potentials of the electronic devices not only can the transmitting range of thezfilter be altered at will, but also the gain or,

if desired, the attenuation of the filter may simultaneously and independently be. adjusted. Since the variation of biasing potentials requires substantially no power, the filter characteristics may be automatically controlled in accordance; for example, with the strength of the signalstraversing the filter.- While these filters differ from ordinary filters in that thereis no reactive coupling between elements, their use is subject to .the same considerations and will be readily understood by one skilled in the art of wave filters.

The novel features which I believe to be characteristic of'my invention are set forth in pariticularity in the appended claims; the invention itself, however, as to both its organization and method of operation'will best be understood by reference to the following description'taken in connection with the drawings inwhich I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

Fig. 1 shows schematically a'filter section utiliz- J'ng reactance elements, transconductances and transimpedances which are dissimilar,

Fig. 2 shows a high pass filter section utilizing similar reactance elements and a pair of screen grid'tubes as transconductances, V Fig. 3 shows aband pass filter section utilizing similar reactance networks, and a pair of screen out reaction of any sorton'the first circuit.

Fig. 5v shows an element of a'section 'of a bandpass filter adapted to have increased uniformity of iterative admittance over a large portion of the transmittingband, and a high degree of relative attenuation at a pair of frequencies on eitherside of the band, and v Fig. 6 shows a receiving system embodying band pass filters constructed in accordance with the invention. I

Referring now to Fig. 1, there is shown a four terminal network terminated by an impedance 7c. The shunt elements 21 and 22 are pure reactance networks vof any nature. The dotted rectangle Y represents transconductance devices carrying out the following function: A positive potential 'e1 at the left side of Y causes current eiyiz to flow into the rectangle fromuthe right without any corresponding fiowoutward from the left; At the same time-a positive potential e2; on the right side of Y causes current ezyzi to flow out of the rec' tangle from the left. Inna similar mannerrectangle Z represents transimpedance devices; that is, devices for introducinginto a circuit a voltage proportional to current in another circuit, with- In the diagram where 212 is the voltage inducedin 22 by unit current in z1,.and 221 is the voltage induced in.z ..by unit current in 22, the arrows within the rectangle indicate the direction of the voltages induced when-the directions of -the currents are as shown by the solid headed arrows. 1

' The iterative impedance is of the four terminal structure is found asinthe case of anordinary filter section; by solving for the input impedance e1/z'1 with is considered as an arbitrary terminating impedance, and then equating this input imfound 7c, the current or voltage ratio per section of .a filter-terminated by its iterative impedance 7c isfound by calculating the ratio ez/ei or iz/z'i,

and substitutingin the expression for either .of these ratios the value of 76 already found. The carrying out of the calculation above outlined .is

simple but tedious, and it will be sufiicien't'here to note that as a result there is secured the fol lowing:

grid tubes so connected as to perform the functions of transimpedances,

Fig. 4 shows a band pass filter employing similar reactance networks and a pair of transcon- Now since both 21 and 22 are pure reactances,

the first term in the denominator ofEquation 2 is pure imaginary. On the other hand, all the quantities under the radical sign of Equation 2' are real. Hence,,for all values of zi-and 22 which 1 pedance to k. This equation defines it. Having I make the quantity under the radical a positive real, the denominator of Equation 2 consists of an imaginary term plus a real term, and the absolute value of the denominator is the square root of the sum of their squares. If Equation 2 is by this method reduced to its absolute value it becomes:

(Z1Z2+Z1ZZ21)Y21+Z21 (Eq' 3) which is the current, or voltage, ratio per section for the range of frequencies specified by requiring the quantity under the radical of Equation 2 to be positive.

To provide a true filter action the ratio given by Equation 3 should be independent of frequencies over a range of frequencies. If ya and 1/21 and 212 and 221 are each independent of frequency, the ratio is also independent if yizzzi -yziziz 01 if y12=y21=0 or if .z12=Z21=0. The first of these conditions leads to structures having two transmission bands, or, as a special case, a structure amplifying all frequencies uniformly. The second and third conditions, howfever, lead to structures much simpler, and of more practical importance, some of which will be described more in detail.

Fig. 2 shows a high-pass. structure where ziz=zzi=0 and z1=zz=1/y. In this case the current ratio per section is simply:

Y+1/Y +Y12Y21 which is constant for all frequencies from infinity (where y=0)' down to the frequency at which i ]Y|=1 .V12. 21 and throughout the range of frequencies so defined has an absolute value In this structure the tube I providing the transconductance 1112 may be an ordinary screen grid tube, while the negative transconductance device 2 required to act as 1121 may be obtained by using a pliodynatron whose plate potential is adjusted to a point of maximum reversed plate current so that the internal plate resistance is extremely high, while the transconductance between control grid and plate current is negative.

Of course, in any of the filters of the invention any suitable negative transconductance device may be used. In fact, an ordinary tube may be used if means are provided for exciting its grid with voltage proportional to c2 of Fig. 1, but opposite in phase from (22. However, as the particular devices that may be used in the electronic portions of the filter are not part of the present invention, they will be indicated merely symbolically by a circle including a control element and a current electrode. Whatever kind of device is used, means for supplying operating and control voltages, and for blocking off undesired direct current potentials where necessary, will be understood to be supplied in any of thewell known ways.

while the gain per section is 1/712/321 this filter may be controlled both as to gain and cut-off by suitable variation of :lj12and yzi. If capacitive admittances are substituted for the inductive admittances y of Fig. 2, the filter becomes a low pass filter, and may advantageously be used in the audio frequency system of a radio receiver where the audio frequency gain and cutoff frequency, or both, are desired to be controlled automatically.

For example, in receiving weak signals accompanied by considerable noise it is desirable to lower the cut-off frequency of the audio system. This may be done automatically by controlling either 11,12 or 1/21, or both, by a voltage derived from rectification of the incoming carrier. If both 1112 and 1121 are varied by similar factors, the gain will remain unaltered. However, if a control voltage developed from rectification of the audio output -of the detector is applied to superpose an opposite variation of rm and J21,

the band Width will not be affected by this superposed variation but the gain will be altered in accordance with -the average strength of audio signals. This control may be used either for compressing or expanding the range of audio intensities.

Fig. 3 shows a low impedance band pass filter where z1=z2=z, yrz=y21=0 and 212 and 221 are independent of frequency. Both the tubes l and 2 and ordinary screen grid tubes. The voltage on the grid of tube I is directly proportional to input current and inversely proportional to frequency, while the induced voltage in the output circuit is directly proportional to plate current of tube l and to frequency. Likewise, the voltage on the grid of tube 2 is proportional to output current and inversely to frequency While the voltage induced in the input circuit by the plate current of tube 2 is proportional to its current and to frequency. Hence, the induced voltage in either circuit is proportional to current in the other circuit, and independent of frequency. As a practical matter, to avoid increasing reactive coupling through unavoidable tube capacity, it is advisable to connect the grid of the tube 2 acting as .221 across only a fraction of the total capacity reactance of the output branch as shown in Fig. 3. In' this case, from Equations 1, 2 and 3 we find v v v w/z -lrz zl and the current ratio per section is Z21 over the band of frequencies Where 2 representing the input circuit impedance and also the output circuit impedance as mentioned above;

Fig. 4 shows a high impedance band pass filter where z1=zz=1/y and z12=z21=0, and ordinary tubes are used. The effect of negative transconductance'for gel is simulated by exciting the grid of the tube 2 providing 1 21 with a voltage I 186 out of phase with the output voltage by means of a reversing transformer winding 3. In this case the iterative admittance i 'Jy 'l-ylzyzi and the current, or voltage ratio per section is over the band of frequencies wherein lY| w y12y21 Fig. 5 shows a particular structure that may Fig. 4 to provide a more uniform'value of iterative impedance over the transmitted band together with high relative attenuation of frequencies just outside of the band. The high attenuation is obtained by arrangingthe structure to be series resonant at frequencies just outside the band on either side'thereof. The uniform value of 1/70 is obtained by makingthe admittance of the structure low'relativeto that the admittance rises rapidly near cut-off to the value which it must have for cut-off."

In Fig. 6 there is shown. a superheterodyne type of radio receiver which. employs an I. F. amplifier consisting of a two section band pass filter; the filter sections being essentially those of Fig. 4. The amplifier tube H! has its input electrodes coupled across the resonant input circuit H, the latter being tuned to the operating I. E. which may be chosen from a range of '75 to 450 k. c. The I. F. energy source may be any ,desired type of converter, or first detector, network; those skilled in the art are fully aware of the superheterodyne type of construction wherein the I. F. amplifier is preceded by one, or more, stages of tunable radio frequency amplification and a first detector; The resonant cir cuit l2, tuned to the I. F. is connected between the output electrodes of the amplifier Ill. The negative transconductance element of the first filter section comprises the tube l3, whose anode .is connected to the high alternating potential side of input circuit H.

maintained at the desired positive voltage by- The anode of tube I3 is connecting the low alternating side of input circuit II to the source of positive voltage B (not shown). A direct current blocking condenser i4 is connected between the grid of tube Wand the anode connection to circuit I I. The cathodes of tubes NJ and I3 are grounded, and

initial negative grid biasing sources l5 .and I6 furnish the grid biases for tubes respectively.

The input electrodes of tube l3 are coupled to the circuit i2 by means of the reversing transformer H; the secondary Winding ll of the latter being connected in series, between the grid "of tube l3 and the negative terminal of bias source 15, with a pulsating current filter resistor l8. The section comprising circuits H, I?! and tubes it], I3 provides a high impedance band pass filter. The grid of tube I3 is excited with a voltage 180 degrees out of phase with the output voltage developed across circuit l2 by virtue of the reversing transformer l'i. Hence, there is simulated the effect of negative transconductance between the input and output circuits II, 12.

The following band pass filter section includes amplifier l9 whose input electrodes are connected across the circuit 12. The negative transconduotance element for this second sectionis Ill and "provided by tube 28 having its grid connected to the secondary winding 22' of reversing transformer 22. in series between the winding 22 and the negative terminal of bias source E6. The anode-of tube 20 is connected to the positive voltage source The filter resistor l8 is connected B' through' th'e coilfof circuit l2. The =output circuit w2I..is tunedto the operating'I. F., and it is connected between the output electrodes of amplifier-19. The sources I5 and I6 provide the and .20 respectively; the cathode of the latter tubes being grounded. The reactances of circuit I 2 are one half as large asuthe reactances of the end circuits H and 2| of the filter.

The terminating resistance 'of'the filter comprisesthe diode rectifiers 24- and 25, the load resistors 24" and 25' thereof, and the audio utilization network. The diode 24 has its anode connectedto the high alternating potential side of circuit 2| through the condenser 26. The diode cathode is at ground potential. The load resistor24' is connected between the electrodes of diode 24, and the audio frequency component of rectifiedLF. current is impressed upon the grid of audio tube 30 through condenser 3| connected to the anode side of resistor 24'. The gain of eachiof" amplifiers I8 and. I9 is varied automatically by connecting the grids of the amplifiers, through proper pulsating current filter resistors .32, to the AVG connection 23' leading to bias source 15. The positive terminal of source- I5 is connected to resistor 24' by an adjustable tap element 40. In this way the magnitude of the AVG bias can be selected; the

source 15 providing the maximum amplification bias for each amplifier l0 and I9.

The diode 25 is connected in reverse manner across circuit 2!. Condenser 4i connects the diode cathode to the terminal 42 of circuit 2|, whereas the diode anode isgrounded. The-load normal negative biases for thegrids of tubes l9 resistor is connected between the electrodes of diode 25'and the audio voltage component of rectified current, flowing through resistorv Z5. is impressed upon the grid of audio tube 150 by condenser 51; the latter connecting the grid of tube 50 to the positive side of resistor 25'; The audio amplifier tubes and5il are connected in push-pull relation; the manner of connecting the diodes 24 and 25 to circuit 2! making it possible to operate the audio amplifier grids in push-pull from the. diode loadresistors.

The gain of each of tubes 53 and Zll'is automatically varied by connecting the ABC lead 23 (these letters designating the automatic band width control circuit) to a desired point on load resistor 25'. The grids of tubes l3-and 20 are connected, through bias source It, to'the adjustable tap element 60, and the latter can be varied to adjust the magnitude of the band width. control bias to be impressed on tubes l3 and 20. The source It provides cut-ofi bias for tubes l3 and 20 in the absence of received signal energy. When signal energy is rectified by diode 25, a positive direct current voltage is developed for overcoming the initial cut-ofi bias due to source I6. The positive bias applied to tubes l3 and 20 increases with signal carrier amplitude increase, and, hence, the transconductance of each tube lsand 20 increases.

The audio output of the push-pull stage 30-58 tive gain control, the tap 40 can be set at an intermediate point on resistor 24'.

most effective band width control is secured by adjusting tap 60 towards the positive end of resistor 25, and setting tap 40 adjacent the oathode terminal of resistor 24'. This is because of the fact that the band width in the filter depends on the square root of the product of the positive and negative transconductances of the two tubes in each section; whereas the amplification of each section depends on the square root of the ratio of said transconductances. From this it follows that moving tap 6D to the upper end of resistor 25', with similar moving of tap '38 to the lower end of resistor 24', results in an increase in the band Widening factor and a decrease in amplification as the signal amplitude increases. The receiver may be operated Without any ABC by moving slider 60 to the grounded end of resistor25'.

The taps 40 and 60 may be actuated in the desired manner by a common adjusting means fili'. However, if freedom of adjustment is desired to secure other relations between band width and amplification, the taps may be permitted to remain independently adjustable. The diodes 26 and 25 may be a tube of the 61-16 type if desired, since the latter comprises a common tube casing housing the electrodes of two diodes. Further, the condensers 26 and 4| may be adjusted in magnitude to maintain a proper impedance match between the filter and its terminating resistance, as the product of the transconductances is varied.

While I have indicated and described several systems for carrying my invention into eifect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but

that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. A receiving system utilizing a band pass filter composed of a number of similar filter elements each consisting solely of a pair of reactance elements substantially free of resistance coupled solely by forward acting and backward acting transconductance elements whose effective transconductances are of opposite signs to each other, an effective terminating resistance for said filter substantially matching its iterative impedance for at least one combination of values of said transconductances, means for rectifying the output of said filter, and connections from said rectified output to at least one of said transconductance elements whereby to control its transconductance in accordance with the level of signals impressed upon said filter, thereby automatically varying both the transmission gain and the band width of said filter in accordance with signals impressed thereon.

2. In a signal carrier receiving system, at least two cascaded resonant circuits tuned to a common carrier frequency, a first electron discharge device having input electrodes connected to the first of said circuits and output electrodes connected to the second of said circuits whereby to provide between said first circuit and said secand circuit an effective transconductance of one sign, a second electron discharge device, also having input electrodes and output electrodes, arranged to provide between the second of said circuits and the first of said circuits an effective transconductance of opposite sign to that of said first mentioned transconductance, the internal impedance between output electrodes of each of said devices being connected in shunt across one of said circuits, means, responsive to an increase in carrier amplitude, for decreasing the transconductance of one device, and means, responsive to said increase, for simultaneously increasing the transconductance of the other device, said two means being in predetermined relation.

3. In a signal carrier receiving system, at least two cascaded resonant circuits tuned to a common carrier frequency, a first electron discharge device having input electrodes connected to the first of said circuits and output electrodes connected to the second of said circuits whereby to provide between said first circuit and said second circuit an effective transconductance of one sign, a second electron discharge device, also having input electrodes and output electrodes, arranged to provide between the second of said circuits and the first of said circuits an effective transconductance of opposite sign to that of said first mentioned transconductance, the internal impedance between output electrodes of each of said devices being connected'in shunt across one of said circuits, a diode means, responsive to an increase in carrier amplitude, for decreasing the transconductance of one device, and a second diode means, responsive to said increase, for simultaneously increasing the transconductance of the other device, said two means being in predetermined relation.

4. In a signal carrier receiving system, at least two cascaded resonant circuits tuned to a common carrier frequency, a first electron discharge device having input electrodes connected to the first of said circuits and output electrodes connected to the second of said circuits whereby to provide between said first circuit and said second circuit an efiective transconductance of one sign, a second electron discharge device, also having input electrodes and output electrodes, arranged to provide between the second of said circuits and the first of said circuits an effective transconductance of opposite sign to that of said first mentioned transconductance, the internal impedance between output electrodes of each of said devices being connected in shunt across one of said circuits, means, responsive to an increase in carrier amplitude, for decreasing the transconductance of one device, and means, responsive to said increase, for simultaneously increasing the transconductance of the other device, said two means being in predetermined relation, said two means including independent rectifiers coupled to one of said resonant circuits.

5. In a modulated carrier receiving system, a filter network comprising a pair of shunt reactive elements substantially free of resistance and resonated to a common carrier frequency, an amplifier tube having its plate to cathode impedance in shunt with one of the reactive elements and its input electrode connected to the other element, a second tube having its plate to cathode impedance in shunt with said other element and its input electrode so coupled with said one element as to receive therefrom a voltage whose magnitude is directly proportional to the voltage thereacross, the said tubes beingarranged to transfer energy in opposite directions between said two elements, means deriving a control voltage from received carrier energy, and means responsive to said control voltage for controlling the gain of said second tube in a sense to widen the pass band of the filter network with carrier amplitude increase.

6. In a modulated carrier receiving system, a filter network comprising a pair of shunt reactive elements substantially free of resistance and resonated to a common carrier frequency, an amplifier tube having its plate to cathode impedance in shunt with one-of the reactive elements and its input electrode connected to the other element, a second tube having its plate to cathode impedance in shunt with said other element and itsinput electrode so coupled with said one element as to receive therefrom a Voltage whose magnitude is directly proportional to the voltage thereacross, the said tubes being arranged to transfer energy in opposite directions between said two elements, means deriving a control voltage from received carrier energy, means responsive to said control voltage for controlling the gain of said second tube in a sense towiden the pass band of the filter network with carrier amplitude increase, means deriving a second control voltage from carrier energy, and means for reducing the gain of said amplifier tube with the second voltage.

'7. In a modulated carrier receiving system, a filter network comprising a pair of shunt reactive elements substantially free of resistance and resonated to a common carrier frequency, an amplifier tube having its plate to-cathode impedance in shunt with one of the reactive elements and its input electrode connected'to the other element, a second tube having its plate to cathode impedance in shunt with said other element and its input electrode so coupled with said one element as to receive'therefrom a voltage whose magnitude is directly proportional to the voltage thereacross, the said tubes being arranged to transfer energy in opposite directions between said two elements, means deriving a control voltage from received carrier energy, means responsive to said control voltage for controlling signal feedback path between said circuits for feeding signal energy from the second of the circuits to the first circuit in degenerative phase with respect to the signal feed through the said tube, a rectifier circuit producing a direct current voltage from signal energy, a second rectifier circuit producing a second direct current voltage from signal energy, means for applying the first voltage to said tube'to control the gain thereof in a sense inverse to signal amplitude change, means for controlling the magnitude of feedback through said path with the second voltage, and means for adjusting the said two voltages in the same polarity sense for controlling the magnitudes thereof.

WALTER VAN B. ROBERTS. 

